249. The quality of the blood may be also essentially changed by the disturbance of the balance of certain organic functions: digestion, absorption, circulation, respiration, are indispensable to the formation of the blood and to the nourishment of the tissues. Absorption, nutrition, secretion, circulation, render the blood impure, either by directly communicating to it hurtful ingredients, or by allowing noxious matters to accumulate in it, or by destroying the relative proportion of its constituents. Organs are specially provided, the main function of which is to separate and remove from the blood these injurious substances. Organs of this class are called depurating, and the process they carry on is denominated that of depuration. The lungs, the liver, the kidneys, are depurating organs, and one result at least of the functions they perform is the purification or depuration of the blood. If the lung fail to eliminate carbon, the liver bile, the kidney urine, carbon, bile, urine, or at least the constituents of which these substances are composed, must accumulate in the blood, contaminate it, and render it incapable of duly nourishing and stimulating the organs. 250. But though the blood be good in quality and just in quantity, health and life must still depend upon its proper distribution. It may be sent out to the system too rapidly or too slowly. It may be distributed to different portions of the system unequally; too much may be sent to one organ, and too little to another: consequently, while the latter languishes, the former may be oppressed, overwhelmed or stimulated to violent and destructive action. In either case health is disturbed and life endangered. 251. Of the mode and degree in which food, air, moisture, temperature, repletion, abstinence, exercise, indolence, influence the quantity, quality, and distribution of the blood; of the mode in which the condition of the blood modifies the actions both of the organic and the animal organs; of the reason why health and disease are wholly dependent on those states and actions, a clear and just conception may be formed when the several functions have been described, and the precise office of each is understood. CHAPTER VII. OF THE CIRCULATION. Vessels connected with the heart: chambers of the heart—Position of the heart—Pulmonic circle: systemic circle—Structure of the heart, artery, and vein—Consequences of the discovery of the circulation to the discoverer—Action of the heart: sounds occasioned by its different movements—Contraction: dilatation—Disposition and action of the valves—Powers that move the blood—Force of the heart—Action of the arterial tubes: the pulse: action of the capillaries: action of the veins—Self-moving power of the blood—Vital endowment of the capillaries: functions—Practical applications.